Of my many sabatoge rounds I've only seen this work once. And that time we were doing so well that we would have easily won with a normal rush.
There's just no reason to be hiding up at C, you'll be much more effective down assisting to take A and B.
Just my 2c.
There is no definitive yes or no on this. Planting at C stops the timer and those 10-20 seconds may give you a win in the end.
But if someone camps C from the get go then yes, it's not a good tactic.
On several occasions, I have ninja'ed C and successfully blown it up myself. Sometimes I only have to fend off 7-8 people before the bomb explodes.
Of course if you're not good at killing people in close quarters--then ninja-planting is obviously a useless idea.